The present invention involves regulation of a gas exhausted through a propeller. The regulation is dependent on a predetermined speed or range of revolutions per minute (rpm) of the propeller.
As disclosed in U.S. Pat. No. 3,279,415 to Kiekhaefer and U.S. Pat. No. 4,511,339 to Kasschau, it is known to have a hollow hub propeller for discharging engine exhaust. This exhaust system is used for outboard motors, stern drive units and the like. Exhaust from the engine is directed rearwardly into the water through an annular casing formed between an inner and outer propeller hub.
Cavitation is the formation of gas- or vapor-filled cavities in liquids in motion which is produced when the pressure is reduced to a critical value while the ambient temperature remains constant. If the velocity of the flowing liquid exceeds a certain value, the pressure is reduced. It is at this point that cavitation occurs, causing a restriction on the speed at which hydraulic machinery can be run without noise, vibration, erosion of metal parts, or loss of efficiency.
As the speed of an object, such as the blades of a rotating propeller, moving through a liquid is increased, there will be a value at which cavitation bubbles will form. During cavitation, if a collapsing bubble is close to metal, the impact can cause mechanical destruction so that the metal becomes pitted or eaten away. Cavitation is accompanied by noise that ranges from a low rumble to loud knocks. The formation and collapse of the bubbles requires energy, so that its presence in fluid machinery produces a drop in efficiency. The possibility of cavitation is reduced as the total pressure in the fluid is increased, its temperature reduced, or its velocity decreased.
In U.S. Pat. No. 3,947,151 to Stillerud et al, a hollow hub marine propeller is disclosed having an anticavitation groove. An external groove girdles the hub between the propeller blades and a rear hub opening. The walls of the groove or annular cavity do not extend outwardly of the hub and thereby offer minimal water resistance. The groove serves to minimize forward migration of exhaust gas issuing from the rear end of the hub, thus to minimize blade cavitation.
Also disclosed in the patent to Stillerud et al are radially extending9 holes emanating from the bottom of an anticavitation groove and extending to an internal passageway of the hub. The holes are described as not usually being desired but under some conditions may serve to assist in the release of water from the groove into the volume of reduced pressure. It is stated that insofar as the holes permit escape of exhaust gas from the hub interior into the volume of reduced pressure, such holes would, however, tend to defeat the purposes of the Stillerud et al invention.
In U.S. Pat. No. 3,788,267 to Strong, anticavitation means for a marine propulsion device are disclosed. In the Strong patent, cavitation eminating from the leading edge near the hub of a propeller of a marine propulsion device is prevented by introducing exhaust gas or air adjacent the junction of the leading edge of each blade of the propeller and the propeller hub.
Strong discloses one or more holes spaced axially along the root of a blade which is radially mounted on an outer hub. The holes provide a source of gas for delivery to the area of each of the blades in which a sufficiently low pressure would exist to cause cavitation bubbles to develop. Due to relief of the vacuum by the gas flowing through the holes, no cavitation bubbles are produced so that there are no cavitation bubbles collapsing on the face of the propeller blades.
Aeration holes are not to be confused anti-cavitation holes, large aeration holes will promote ventilation at low speed and cavitation at high speed, whereas small anti-cavitation holes prevent only cavitation.